In a microwave plasma processing apparatus, in order to generate plasma by exciting a gas, a desired microwave as energy for ionizing or dissociating the gas is transmitted to a transmission path, and is emitted into a processing container. In the plasma processing apparatus having a structure wherein a gas passage is formed on a ceiling surface of the processing container and the gas is introduced into the processing container from the ceiling surface by using the ceiling surface as a shower plate, a thickness of a coaxial waveguide is difficult to be increased since a gas passage has to be formed in a location that does not interfere with the coaxial waveguide for transmitting the microwave.
Meanwhile, a considerable amount of power is required to stably generate uniform plasma. For example, since hundreds of W to several kW of microwaves have to be transmitted to the coaxial waveguide so as to excite high density plasma, a current density of an inner conductor of the coaxial waveguide is increased, and thus the inner conductor is heated up. When the inner conductor of the coaxial waveguide is heated up, a Teflon® ring or the like holding the inner conductor may be deformed or deteriorated, a surface of the inner conductor may be oxidized, or the inner conductor may be damaged as the inner conductor extends and stress is applied to each portion of the inner conductor.
In order to supply microwaves having large power to plasma while preventing the coaxial waveguide from being damaged, characteristic impedance of the coaxial waveguide is required to be optimized in each portion so as to suppress reflection of the microwaves to a minimum. However, in a conventional plasma processing apparatus, a coaxial waveguide having a simple structure and a uniform thickness is used to supply microwaves into a processing container (for example, refer to Patent Document 1). Accordingly, characteristic impedance is uniform along a longitudinal direction of the coaxial waveguide.    (Patent Document 1) Japanese Laid-Open Patent Publication No. hei 9-106900